In order to efficiently perform its function of a pump, the heart must maintain a natural A-V synchrony. The term "A-V synchrony" relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, the atria (A) contract prior to the ventricles (V) in accordance with a prescribed timing or synchronized relationship, hence the term "A-V synchrony." These contractions are typically manifest or measured by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram (ECG) and include a P-wave, representing the depolarization of the atria; the QRS-wave (sometimes referred to as a R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and masked out by the much larger QRS-wave on the ECG.)
Thus, it is the P-QRS-T cycle of waves that represent the natural A-V synchrony of the heart. These waves, including the timing relationships that exist therebetween, are carefully studied and monitored through conventional ECG techniques whenever the operation and performance of the heart is being examined.
A pacemaker is a medical device that assists the heart in maintaining a desired A-V synchrony by monitoring the atria and/or ventricles for the occurrence of P-waves and/or R-waves, and by producing stimulation pulses that are delivered to an appropriate chamber of the heart to cause that chamber to depolarize, and hence contract. (Because the main function of the pacemaker is to provide such stimulation pulses, a pacemaker is frequently referred to as a "pulse generator.") If, for some reason, the heart is unable to maintain its natural A-V synchrony, a pacemaker is utilized to monitor the heart and to provide electrical stimulation pulses when it senses that the heart is not maintaining proper A-V synchrony. A dual-chamber pacemaker, for example, monitors both the right atrium and right ventricle. If it senses an atrial depolarization at appropriate times, no atrial stimulation pulse is generated. If it senses a ventricular depolarization within a prescribed time after the atrial depolarization, no ventricular stimulation pulse is generated. If however, it fails to sense either the atrial or ventricular depolarization within prescribed time periods, then stimulation pulses, frequently referred to as an A-pulse (if delivered to the atrium) and/or a V-pulse (if delivered to the ventricle), are generated and delivered to the appropriate chamber of the heart at an appropriate time in order to maintain the correct heart rhythm.
One of the problems that complicates the operation of a dual-chamber pacemaker, i.e., one that is capable of sensing and/or pacing in both chambers of the heart, is "retrograde conduction." Retrograde conduction is a condition when the depolarization of the ventricles propagates backwards into the atria, causing the atria to depolarize prematurely. This atrial depolarization is manifest by the occurrence of a P-wave, frequently referred to as a "retrograde P-wave". A retrograde P-wave appears on the ECG substantially the same as a natural P-wave except that it occurs much too soon after a ventricular contraction. (A "natural" P-wave results from the natural A-V synchrony of the heart as set by the heart's natural sinus rhythm, and is hereafter referred to as a "sinus" P-wave.) See U.S. Pat. No. 4,788,980 for a more thorough description of retrograde conduction.
Unfortunately, the pacemaker sensing circuits cannot readily distinguish between a retrograde P-wave and a sinus P-wave. A significant problem thus exists because once a P-wave is sensed, the pacemaker (depending upon its mode of operation) will typically generate a V-pulse a prescribed delay thereafter, referred to herein as the "P-V delay," unless an R-wave is sensed during the P-V delay. (It is noted that much of the literature refers to the P-V delay, as that term is used herein, as the "A-V delay," or AVD. Further, some pacemakers employ one delay, a P-V delay, following a P-wave, and another slightly different delay, or AV delay, following an A-pulse. For purposes of the present invention, all such delays following an atrial event, whether an A-pulse or P-wave, are referred to herein as the "P-V delay.") If the sensed P-wave is a retrograde P-wave, an R-wave will not likely occur during this relatively short P-V delay time interval because the contraction of the ventricles just occurred prior to the retrograde P-wave. Thus, at the conclusion of the P-V delay, a V-pulse is generated by the pacemaker, causing the ventricles to again contract, which contraction causes another retrograde P-wave. This retrograde P-wave, in turn, causes another V-pulse to be generated after the P-V delay, causing the cycle to repeat, resulting in a pacemaker mediated tachycardia, or PMT. (A "tachycardia" is a very rapid rhythm or rate of the heart.)
Note that during a PMT, it is the pacemaker itself that causes or "mediates" the tachycardia by tracking each P-wave caused by the retrograde conduction, and providing a ventricular stimulation pulse a programmed P-V delay thereafter. The pacemaker thus provides the forward conduction path (from the atria to the ventricles) electronically by tracking each P-wave and generating a V-pulse (ventricular stimulation pulse) if no R-wave is sensed within a prescribed time thereafter (the programmed P-V delay). The reverse or backward conduction path (from the ventricles to the atria) is provided by retrograde conduction originating with the depolarization of the ventricles, which depolarization occurs as a result of the V-pulse. Thus, retrograde conduction passes the ventricular depolarization back to the atria, causing the atria to depolarize (resulting in a retrograde P-wave), and the process repeats.
Unfortunately, a PMT can be triggered by numerous events. The most common mechanism for triggering a PMT is a premature ventricular contraction, or PVC. A PVC, in turn, is not an uncommon occurrence for most mammalian hearts. A cough or a sneeze, for example, may cause a PVC. Unfortunately, for a patient having a dual-chamber pacemaker that is set to operate in a mode that tracks P-waves and stimulates the ventricle, the occurrence of a single PVC can reset the pacemaker timing in a manner that allows the pacemaker to begin tracking retrograde P-waves, causing a PMT to occur. Such PMT, if allowed to continue for more than just a few cycles, seriously impacts the ability of the heart to efficiently perform its function of a pump. What is needed, therefore, is a system or method for accurately detecting the occurrence of a PMT and quickly terminating such PMT once detected.
One common technique used to prevent a PMT is to first detect a PVC, and assume that any rapid heart rate thereafter is a PMT. Thus, in order to prevent the occurrence of a PMT, it is known in the art for a pacemaker, upon the detection of a PVC, to revert to a DVI mode of operation for one cycle. (For an explanation of the various pacemaker modes--DDD, DDI, DVI, VVI, etc.--see, e.g., U.S. Pat. No. 4,712,555.) This response, in effect, turns off the atrial sense amplifiers for one cycle. Hence, subsequent to the detection of the PVC, no P-waves can be sensed by the pacemaker because the electronic sense circuits are masked from sensing any atrial events, whether a retrograde event or a normal event. It is thus not possible for the pacemaker to generate a V-pulse one P-V delay after a retrograde P-wave, because the retrograde P-wave is not sensed. If the retrograde P-wave is not sensed, the occurrence of a PMT is prevented.
One problem with this approach of turning off the atrial sense amplifiers for the one cycle is that if during the one cycle response using the DVI mode a normal sinus rhythm with spontaneous R-wave occurs, the PVC response remains on because the pacemaker interprets the spontaneous R-wave as another PVC. Thus, even though a possible PMT is prevented, the loss of normal P-wave tracking may occur because P-waves are masked by the response to the detected PVC, and any R-waves that are detected are interpreted as another PVC. Hence, the PVC response may become "stuck," as there is no way for it to terminate. Loss of P-wave tracking may occur from seconds to hours depending on the pacemaker's programmed rate settings and the patient's sinus rate.
Another technique known in the art aimed at preventing a PVC from triggering a PMT, is to extend the Post Ventricular Atrial Refractory Period (PVARP) by a prescribed amount, such as 480 milliseconds, upon the detection of a PVC, thus masking retrograde conduction during this period of time. In addition, the atrial escape interval (V-A delay) is fixed to a prescribed value, such as 830 milliseconds, regardless of the programmed or sensor indicated rate (if a sensor is used, such as is the case in a rate-responsive pacemaker). See, e.g., U.S. Pat. No. 4,788,980. The difference between the selected PVARP value and the fixed VA delay, which difference is 350 milliseconds for the example given, advantageously allows a "window of time" during which a P-wave may be detected.
The extended PVARP approach described above is an improvement over the DVI on PVC approach because the extended PVARP interval is sufficient to mask most retrograde conduction in the majority of patients, and P-waves not related to retrograde conduction can still be tracked. However, unless the sinus P-wave or other atrial event (e.g., an A-pulse) occurs during the window of time defined subsequent to the extended PVARP interval and prior to the termination of the VA delay (e.g., during the 350 milliseconds time period for the example times given above), the PVC response continues. Unfortunately, the PVC response can continue if P-waves fall within the extended PVARP interval (which will not be detected) followed by R-waves that cause the VA delay interval to be reset (with the R-waves being interpreted as PVCs). When this occurs, the PVC response thus causes a fixed atrial escape interval. In turn, this results in a slowdown of ventricular rate because the rate of pacing is made up of the A-V delay (P-V delay) and the atrial escape interval (V-A delay). Such a reduced ventricular rate may not meet the patient's then-existing physiological needs. What is needed, therefore, is a system that in its attempt to prevent a PMT does not slow down the patient's ventricular rate for a prolonged period.
Another technique known in the art for recognizing and breaking a PMT is used by the SYNCHRONY.RTM. pacemaker, manufactured by Siemens-Pacesetter, Inc., Sylmar, Calif. The SYNCHRONY.RTM. pacemaker utilizes both a maximum tracking rate (MTR) and a tachycardia recognition rate (TRR). The TRR is less than or equal to the MTR. Whenever the SYNCHRONY.RTM. pacemaker is pacing in the ventricle as a result of tracking P-waves and senses a rate that is higher than the TRR, the tachycardia termination routine is activated. This tachycardia termination routine operates as follows: following the 10th-127th beat at a heart rate greater than the TRR, the PVARP is extended to approximately 500 milliseconds. This is a sufficient extension to prevent most retrograde P-waves from being sensed, since most retrograde P-waves occur within 250-400 milliseconds after the contraction of the ventricle. Following the 500-millisecond PVARP, there is an approximately 350-millisecond alert period during which the pacemaker is able to sense a sinus P-wave. If no P-wave occurs by the end of this 350-millisecond alert period, the pacemaker logic circuits cause an atrial stimulation pulse, or A-pulse, to be generated. In either event (i.e., whether a P-wave is sensed or an A-pulse is generated), this should be the end of the PMT. This method of terminating a PMT is described more thoroughly in U.S. patent application Ser. No. 07/491,385, filed 03/09/90, which application is assigned to the same assignee as is the present application, and which application is incorporated herein by reference.
Unfortunately, while the PMT termination approach described above operates to terminate most PMT's, there are some situations where this is not the case. For example, if a ventricular beat (R-wave) is sensed before the sensed sinus P-wave or the delivered A-pulse, the pacemaker logic causes PVARP to remain extended for another cardiac cycle, thereby rendering the pacemaker incapable of sensing P-waves for an additional 500 millisecond period. This extended PVARP of 500 milliseconds continues for each cardiac cycle where an R-wave is sensed before a P-wave. Thus, as R-waves continue to be sensed, it is possible for PVARP to be continually extended, thereby effectively eliminating any capability of the pacemaker to sense and track P-waves (because P-waves cannot be sensed during the 500 milliseconds after a ventricular contraction). What is needed, therefore, is an improved response to a sensed PMT that is not extended indefinitely.
An additional problem is created whenever PVARP is extended when the pacemaker is a rate-responsive pacemaker. In a rate-responsive pacemaker, the pacing rate is controlled by a separate activity sensor that detects patient activity (or some other parameter indicative of the need to adjust the heart rate). If such an activity sensor is employed, and if the extended PVARP response continuously repeats (i.e., R-waves are sensed but P-waves are not), then, in effect, the activity sensor is disabled. For example, if the P-V delay is 150 milliseconds, then the rate during an extended PVARP response would be, using the same numbers presented above, 150 milliseconds+850 milliseconds=1000 milliseconds, or about 60 beats per minute. Any sinus P-waves falling within the extended PVARP interval are not sensed, hence the extended PVARP response remains on when accompanied by detected R-waves prior to the end of the V-A delay. Thus, sensor controlled rates are prevented from being effective since the extended PVARP interval controls the atrial escape interval. In other words, since the extended PVARP response slows down the ventricular rate from a higher sensor controlled rate, it is more likely that a sinus rhythm will keep the extended PVARP response on, thereby causing a slower ventricular rate, which slower rate may be undesirable when the patient may be in need of increased cardiac output during activity. Hence, multiple extensions of PVARP upon the detection of a PMT may be an inappropriate response for a sensor-driven rate responsive pacer. What is needed, rather, is a technique or method for clearly recognizing and responding to the occurrence of a PMT, regardless of whether the pacemaker responsible for the PMT is a rate-responsive pacemaker or a fixed (programmable) rate pacemaker.
It is thus evident that there is a need in the art for a system that can terminate a PMT during periods of activity, which PMT response will reduce ventricular rate slowdown during the PMT response, and thus prevent an abrupt change in cardiac output, and allow for P-wave tracking immediately after the PVARP interval. What is further needed is a method of PVARP programming that will allow tracking P-waves at higher rates such as during periods of activity. Programming PVARP long (such as 480 milliseconds during the PMT response, as previously described) will defeat the purpose of tracking P-waves at higher rates and will cause a slowdown in the ventricular rate, as previously described. With an appropriately timed atrial pulse, which pulse is timed from the last P-wave, the PVARP interval does not have to be programmed long as in prior art since the atrial pulse antegrade conduction and P-wave retrograde conduction will extinguish each other and prevent sensing retrograde conduction, which might otherwise be sensed after a PVARP interval which was not extended. The present invention advantageously addresses the above and other needs.